Commercial transport aircraft are typically designed to carry a given load of passengers, cargo, or passengers and cargo over a given range. Occasionally, however, the need arises to increase the range of the aircraft to serve other routes. Increasing the range generally requires increasing the fuel capacity of the aircraft.
Another situation in which it may be necessary to increase the fuel capacity of an aircraft occurs when the role of the aircraft changes. For example, some military aircraft may serve as aerial refueling tankers at one point in time and cargo carriers at another. In the refueling tanker role, auxiliary fuel tanks can be installed in the body (i.e., the fuselage) to increase the amount of fuel that can be off-loaded to other aircraft in flight. In the cargo carrier role, the body tanks can  be removed to increase cargo capacity. Whether auxiliary fuel tanks are added to increase range or to increase fuel off-load capacity, they should be relatively easy to install and remove so that the aircraft can be quickly changed into the desired configuration.
One known type of auxiliary fuel tank system includes an auxiliary tank installed in a fuselage of an aircraft. The system uses pneumatic pressure to transfer fuel from the auxiliary tank to a center-wing tank of the aircraft. The source of the pneumatic pressure can be cabin air. Alternatively, a supplemental blower system can be used to deliver pneumatic pressure when the cabin air is not sufficient to transfer the fuel. This particular auxiliary fuel tank includes double-wall construction.
Another known type of auxiliary fuel tank system includes a group of three tanks linked together in a fuselage of an aircraft in a cascading fill/empty arrangement. Like the system described above, this system also uses pneumatic pressure to transfer fuel from the auxiliary tanks to a center-wing tank of the aircraft. In this system, however, the separate tanks are filled in sequence with the first tank overflowing into the next and continuing until all the tanks are full. Fuel is transferred out of the tanks in reverse. That is, the last tank empties first and then the next tank until all of the tanks are empty. The first tank in the group to fill is connected to the main fuel system of the aircraft. The last tank in the group to fill is connected to the aircraft vent system and the pressurization source.
A further known type of auxiliary fuel tank system includes a group of three tanks having individual fuel inlet, fuel outlet, and vent manifolds. Each tank includes individual valves to control the inflow and outflow of fuel from the tank. In addition, a single electric motor-driven fuel pump can be installed in each tank for transferring fuel out of the tank. Alternatively, pneumatic pressure from an aircraft bleed air system can be individually provided to each of the tanks for fuel transfer.
Yet another known type of auxiliary fuel tank system includes two or more auxiliary tanks ganged together with slip-together, low-level interconnects that  maintain a uniform fuel level across all the tanks. Fuel is added to the tanks via a main fueling manifold of the aircraft. Pneumatic pressure from an aircraft bleed air system is used to flow fuel from the auxiliary tanks into integral aircraft fuel tanks. Venting of the auxiliary tanks is provided via existing aircraft fuel system vents.
A further known type of auxiliary fuel tank system can be found on KC-135 series aircraft. This system uses a number of flexible bladders that are permanently laced into a lower section of the fuselage structure. The bladders include low-level interconnects that allow fuel to migrate from one bladder to the next. An aircraft fueling manifold provides fuel to the bladders for filling. Motor-driven pumps are used to move fuel out of the bladders and return it to the aircraft fuel system or to an aerial refueling system. In this system, the auxiliary tank structure (i.e., the bladder) is single-wall construction.